1. Technical Field
The invention relates to optical devices and is especially applicable to waveguide structures and integrated optics.
2. Background Art
This specification refers to several published articles. For convenience, the articles are cited in full in a numbered list at the end of the description and cited by number in the specification itself. The contents of these articles are incorporated herein by reference and the reader is directed to them for reference.
In the context of this patent specification, the term xe2x80x9coptical radiationxe2x80x9d embraces electromagnetic waves having wavelengths in the infrared, visible and ultraviolet ranges.
The terms xe2x80x9cfinitexe2x80x9d and xe2x80x9cinfinitexe2x80x9d as used herein are used by persons skilled in this art to distinguish between waveguides having xe2x80x9cfinitexe2x80x9d widths in which the actual width is significant to the performance of the waveguide and the physics governing its operation and so-called xe2x80x9cinfinitexe2x80x9d waveguides where the width is so great that it has no significant effect upon the performance and physics or operation.
At optical wavelengths, the electromagnetic properties of some metals closely resemble those of an electron gas, or equivalently of a cold plasma. Metals that resemble an almost ideal plasma are commonly termed xe2x80x9cnoble metalsxe2x80x9d and include, among others, gold, silver and copper. Numerous experiments as well as classical electron theory both yield an equivalent negative dielectric constant for many metals when excited by an electromagnetic wave at or near optical wavelengths [1,2]. In a recent experimental study, the dielectric function of silver has been accurately measured over the visible optical spectrum and a very close correlation between the measured dielectric function and that obtained via the electron gas model has been demonstrated [3].
It is well-known that the interface between semi-infinite materials having positive and negative dielectric constants can guide TM (Transverse Magnetic) surface waves. In the case of a metal-dielectric interface at optical wavelengths, these waves are termed plasmon-polariton modes and propagate as electromagnetic fields coupled to surface plasmons (surface plasma oscillations) comprised of conduction electrons in the metal [4].
It is known to use a metal film of a certain thickness bounded by dielectrics above and below as an optical slab (planar, infinitely wide) waveguiding structure, with the core of the waveguide being the metal film. When the film is thin enough, the plasmon-polariton modes guided by the interfaces become coupled due to field tunnelling through the metal, thus creating supermodes that exhibit dispersion with metal thickness. The modes supported by infinitely wide symmetric and asymmetric metal film structures are well-known, as these structures have been studied by numerous researchers; some notable published works include references [4] to [10].
In general, only two purely bound TM modes, each having three field components, are guided by an infinitely wide metal film waveguide. In the plane perpendicular to the direction of wave propagation, the electric field of the modes is comprised of a single component, normal to the interfaces and having either a symmetric or asymmetric spatial distribution across the waveguide. Consequently, these modes are denoted sb and ab modes, respectively. The sb mode can have a small attenuation constant and is often termed a long-range surface plasmon-polariton, The fields related to the ab mode penetrate further into the metal than in the case of the sb mode and can be much lossier by comparison. Interest in the modes supported by thin metal films has recently intensified due to their useful application in optical communications devices and components. Metal films are commonly employed in optical polarizing devices [11] while long-range surface plasmon-polaritons can be used for signal transmission [7]. In addition to purely bound modes, leaky modes are also known to be supported by these structures.
Infinitely wide metal film structures, however, are of limited practical interest since they offer one-dimensional (1-D) field confinement only, with confinement occurring along the vertical axis perpendicular to the direction of wave propagation, implying that modes will spread out laterally as they propagate from a point source used as the excitation. Metal films of finite width have recently been proposed in connection with polarizing devices [12], but merely as a cladding,
In addition to the lack of lateral confinement, plasmon-polariton waves guided by a metal-dielectric interface are in general quite lossy. Even long-range surface plasmons guided by a metal film can be lossy by comparison with dielectric waveguides. Known devices exploit this high loss associated with surface plasmons for the construction of plasmon-polariton based modulators and switches. Generally, known plasmon-polariton based modulator and switch devices can be classified along two distinct architectures. The first architecture is based on the phenomenon of attenuated total reflection (ATR) and the second architecture is based on mode coupling between a dielectric waveguide and a nearby metal. Both architectures depend on the dissipation of optical power within an interacting metal structure.
ATR based devices depend on the coupling of an optical beam, which is incident upon a dielectric-metal structure placed in optical proximity, to a surface plasmon-polariton mode supported by the metal structure. At a specific angle of incidence, which depends on the materials used and the particular geometry of the device, coupling to a plasmon mode is maximised and a drop in the power reflected from the metal surface is observed. ATR based modulators make use of this attenuated reflection phenomenon along with means for varying electrically or otherwise at least one of the optical parameters of one of the dielectrics bounding the metal structure in order to shift the angle of incidence where maximum coupling to plasmons occurs. Electrically shifting the angle of maximum coupling results in a modulation of the intensity of the reflected light. Examples of devices that are based on this architecture are disclosed in references [23] to [36].
Mode coupling devices are based on the optical coupling of light propagating in a dielectric waveguide to a nearby metal film placed a certain distance away and in parallel with the dielectric waveguide. The coupling coefficient between the optical mode propagating in the waveguide and the plasmon-polariton mode supported by the nearby metal film is adjusted via the materials selected and the geometrical parameters of the device. Means is provided for varying, electrically or otherwise, at least one of the optical parameters of one of the dielectrics bounding the metal. Varying an optical parameter (the index of refraction, say) varies the coupling coefficient between the optical wave propagating in the dielectric waveguide and the lossy plasmon-polariton wave supported by the metal. This results in a modulation in the intensity of the light exiting the dielectric waveguide. References [37] to [40] disclose various device implementations based upon this phenomenon. Reference [41] further discusses the physical phenomenon underlying the operation of these devices.
Reference [42] discusses an application of the ATR phenomenon for realising an optical switch or bistable device,
These known modulation and switching devices disadvantageously require relative high control voltages and have limited electrical/optical bandwidth.
The present invention seeks to eliminate, or at least mitigate, one or more of the disadvantages of the prior art.
According to one aspect of the present invention there is provided a waveguide structure comprising a thin strip having finite width and thickness with dimensions such that optical radiation having a wavelength in a predetermined range couples to the strip and propagates along the length of the strip as a plasmon-polariton wave. The strip may compnse a material having a relatively high free charge carrier density, for example a conductor or certain classes of highly-doped semiconductor. The surrounding material may have a relatively low free charge carrier density, i.e. an insulator or undoped lightly doped semiconductor.
Such a strip of finite width offers two-dimensional (2-D) confinement in the transverse plane, i.e. perpendicular to the direction of propagation, and, since suitable low-loss waveguides can be fabricated from such strip, it may be useful for signal transmission and routing or to construct components such as couplers, power splitters, interferometers, modulators, switches and other typical components of integrated optics. In such devices, different sections of the strip serving different functions, in some cases in combination with additional electrodes. The strip sections may be discrete and concatenated or otherwise interrelated, or sections of one or more continuous strips.
For example, where the optical radiation has a free-space wavelength of 1550 nm, and the waveguide is made of a strip of a noble metal surrounded by a good dielectric, say glass, suitable dimensions for the strip are thickness less than about 0.1 microns, preferably about 20 nm, and width of a few microns, preferably about 4 microns.
The strip could be straight, curved, bent, tapered, and so on.
The dielectric material may be inhomogeneous, for example a combination of slabs, strips, laminae, and so on. The conductive or semiconductive strip may be inhomogeneous, for example a gold layer sandwiched between thin layers of titanium.
The plasmon-polariton wave which propagates along the structure may be excited by an appropriate optical field incident at one of the ends of the waveguide, as in an end-fire configuration, and/or by a different radiation coupling means.
The low free-carrier density material may comprise two distinct portions with the strip extending therebetween, at least one of the two distinct portions having at least one variable electromagnetic property, and the device then may further comprise means for varying the value of said electromagnetic property of said one of the portions so as to vary the propagation characteristics of the waveguide structure and the propagation of the plasmon-polariton wave.
In some embodiments of the invention, for one said value of the electromagnetic property, propagation of the plasmon-polaziton wave is supported and, for another value of said electromagnetic property, propagation of the plasmon-polariton wave is at least inhibited. Such embodiments may comprise modulators or switches.
Different embodiments of the invention may employ different means of varying the electromagnetic property, such as varying the size of at least one of said portions, especially if it comprises a fluid.
The at least one variable electromagnetic property of the material may comprise permittivity, permeability or conductivity.
Where the portion comprises an electrooptic material, the variable electromagnetic property will be permittivity, which may be varied by applying an electric field to the portion, or changing an electric field applied thereto, using suitable means.
Where the portion comprises a magneto-optic material, the variable electromagnetic property will be permittivity which may be varied by applying a magnetic field to the portion or changing a magnetic field applied thereto, using suitable means.
Where the portion comprises a thermo-optic material, the electromagnetic property may be permittivity and be varied by changing the temperature of the material.
Where the portion comprises an acousto-optical (photoelastic) material, the electromagnetic property may be permittivity and be varied by changing mechanical strain in the material.
Where the portion comprises a magnetic material (such as a ferrite), the electromagnetic property will be permeability and may be varied by applying a magnetic field to the material or changing a magnetic field applied thereto, by suitable means.
Where the portion comprises a semiconductor material, the electromagnetic property will be conductivity or permittivity and may be varied by changing free charge carrier density in said portion, using suitable means.
Additionally or alternatively, the propagation of the plasmon-polariton wave may be varied by varying an electromagnetic property of the strip. For example, the permittivity of the strip may be varied by changing the free charge carrier density or by changing or applying a magnetic field through the strip.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings, of a preferred embodiment of the invention.